Japan’s coast guard is investigating why it took almost an hour for the deadly collision between a US destroyer and a container ship four times its size ship to be reported.

A coast guard official said Monday that investigators are looking into what the crew of the Philippine-flagged ACX Crystal was doing before reporting the collision off Japan’s coast.

Seven of the USS Fitzgerald’s almost 300 crew members were killed in the collision early Saturday.

Authorities have declined to speculate on a cause while the crash remains under investigation.

A track of the container vessel’s route by the MarineTraffic tracking service shows it made a sudden turn as if trying to avoid something at about 1:30 a.m., before continuing eastward.

It then made a U-turn and returned around 2:30 a.m. to the area near the collision.

Japanese authorities initially said the collision occurred at 2:20 a.m. because the Philippine ship had reported it at 2:25 a.m. and said it just happened. After interviewing Filipino crewmembers, however, the coast guard changed the collision time to 1:30 a.m.

Coast guard official Tetsuya Tanaka said they are trying to resolve what happened during the fateful 50 minutes. He said officials are planning to get hold of a device with communication records to examine further details of the crash.

Japan’s Transport Safety Board also started a probe Monday.

Nanami Meguro, a spokeswoman for NYK Line, the ship’s operator, agreed with the revised timing of the accident.

Meguro said the ship was “operating as usual” until the collision at 1:30 a.m., as shown on a ship tracking service that the company uses.

She said the ship reported to the coast guard at 2:25 a.m. — but she could not provide details about what the ship was doing for almost an hour.

“Because it was in an emergency, the crew members may not have been able to place a call,” she said.

The coast guard officials is investigating the case as possible professional negligence, but no criminal charges have been lodged so far.

On Monday, the Navy’s 7th Fleet identified the seven sailors who died.

She said the family moved to Connecticut was Huynh was in eighth-grade. He graduated from Watertown High School and also attended Naugatuck Valley Community College before enlisting the Navy in 2014. The family moved to Oklahoma a short time later.